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Clarify stability guarantee for lifetimes in enum discriminants #104299

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Sep 8, 2023
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Clarify stability guarantee for lifetimes in enum discriminants #104299

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5 changes: 5 additions & 0 deletions library/core/src/mem/mod.rs
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Expand Up @@ -1118,6 +1118,11 @@ impl<T> fmt::Debug for Discriminant<T> {
///
/// [Reference]: ../../reference/items/enumerations.html#custom-discriminant-values-for-fieldless-enumerations
///
/// The value of a [`Discriminant<T>`] is independent of any *lifetimes* in `T`. As such, reading
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Suggested change
/// The value of a [`Discriminant<T>`] is independent of any *lifetimes* in `T`. As such, reading
/// The value of a [`Discriminant<T>`] is independent of any **free** *lifetimes* in `T`. As such, reading

It can be unsound to transmute Discrimant<MyType<for<'a, 'b> fn(Inv<'a>, Inv<'b>)>> to Discrimant<MyType<for<'a> fn(Inv<'a>, Inv<'a>)>>.

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(as a general procedural thought, I feel like this decision should have been at least co-owned by t-types)

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opened #118006

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@mkrasnitski mkrasnitski Nov 17, 2023 •
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My apologies about not looping in t-types. That team didn't exist when I first opened this PR. However, I agree they should have been added while this PR was still under review.

/// or writing a `Discriminant<Foo<'a>>` as a `Discriminant<Foo<'b>>` (whether via [`transmute`] or
/// otherwise) is always sound. Note that this is **not** true for other kinds of generic
/// parameters; `Discriminant<Foo<A>>` and `Discriminant<Foo<B>>` might be incompatible.
///
/// # Examples
///
/// This can be used to compare enums that carry data, while disregarding
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